Abstract: This paper studies the internal flow field and the performance of the contra-rotating fan of four different distances between two impellers FLUENT. The results show that when distance raising，the efficiency increased first and then decreased; the interference between two impellers reduces the axial distance increasing at a certain range, which helps to homogenize airflow; The mutual interference of the two impellers is mainly reflected in the interference of the first stage impeller to the second stage, the effect of the second stage impeller to the first stage is not as much. The decrease of the axial distance could cause serious vortex in the axial spacing, and increase the eddy current loss. But a large axial distance not only increases the axial size of the fan, but also Increases the energy loss, resulting in the decrease of fan pressure, flow and efficiency .